Equinox brings brief balance as tilted Earth proceeds to fall

August 31, 2008

The autumn equinox, which marks the start of fall, occurs at 5:45 a.m. on Monday, Sept. 22. (The vernal equinox, marking the start of spring, occurred this year on March 19.)

"Equinox" comes from the Latin phrase for "equal night." On these two days, no matter where you are on earth, the sun rises roughly at 6 a.m. and sets roughly at 6 p.m.

Whether near the equator or the poles, whether in the northern or the southern hemisphere, all places on earth get almost exactly 12 hours of day and 12 hours of night on the equinoxes. The only exceptions are the regions around the poles, where the sun just skims the horizon all day long on the equinoxes.

As we head into winter, this worldwide equality of day and night vanishes. On Dec. 21, the first day of winter, we'll get a pleasant 11 hours of daylight in Honolulu, even on the shortest day of the year. In contrast, our friends in Anchorage will have a little more than five hours of daylight on Dec. 21. The residents of Christchurch, New Zealand, on the other hand, get 15 hours of daylight on Dec. 21, the start of the southern hemisphere summer.

If the Earth were not tilted on its axis, every day would be 12 hours long, and every night would be 12 hours long, all the year round and all over the planet.

Instead, the Earth is titled 23.5 degrees. As we orbit the sun in the course of the year, there are times during the year when the northern hemisphere is tilted in towards the sun (summer) and times when the northern hemisphere is tilted away from the sun (winter). That said, we have two times during the year when the tilt of the Earth doesn't matter: the first day of spring and the first day of fall. On these two equinoxes neither hemisphere tilts in, and neither hemisphere tilts away from the sun. At these two times of the year we get a glimpse of what our planet would be like if Earth were not tilted at all.

The 17th-century poet John Milton uses this fact to explain paradise and the loss of paradise. In the world of Adam and Eve before the fall of man, the Earth is straight up and down. All days are 12 hours, all nights are 12 hours, and the weather is spring-like all over the planet.

After Adam and Eve bite the apple, God sends the angels to make one change: they tilt the Earth. With that action come the cold days of winter and the blistering days of summer, and farewell to the paradise of eternal spring. The poem, appropriately, is called "Paradise Lost."

The closer you are to the equator, the less the tilt of the Earth matters. In fact, on the equator, in a city like Singapore or Quito, Ecuador, every day is 12 hours and every night is 12 hours.

As you move away from the equator the change in the length of days becomes more and more striking. In Honolulu, a mere 21 degrees north of the equator, the sun is above the horizon for 13 hours on our longest day (June 21), and for just under 11 hours on the shortest day of the year (Dec. 21). By the time we get to Anchorage, the longest day on June 21 is 19 hours long, and the shortest day on Dec. 21 is a little over five hours long. Once you get above the Arctic Circle (66.5 degrees N), you have days in the winter where the sun never rises, and summertime days where the sun never sets. All of this due to the one simple fact that our planet is tilted 23.5 degrees.

Mike Shanahan, director of education and exhibits at Honolulu's Bishop Museum, can be reached at mikes@bish opmuseum.org.